There are techniques for reducing warpage of printed circuit boards (PCBs). Japanese Laid-Open Patent Publication No. 2006-108460 discloses a technique in which elastic strips are embedded in a PCB to absorb warpage of the PCB. Japanese Laid-Open Patent Publication No. 2007-88293 discloses a technique in which elements having substantially the same dimensions as electronic components are provided on the back side of a PCB on which the electronic components are mounted to reduce warpage of the PCB.
[Patent Document 1]
Japanese Laid-Open Patent Publication No. 2006-108460
[Patent Document 2]
Japanese Laid-Open Patent Publication No. 2007-88293
[Patent Document 3]
Japanese Laid-Open Patent Publication No. 1-291438
The mechanism of how a PCB is warped will be described. Electrodes 13x of an electronic component 10x which are made of solder are arranged on terminals 23x of a PCB 20x as illustrated in FIG. 1A. Then, the electronic component 10x and the PCB 20x are heated to melt the electrodes 13x to join the electrodes 13x to the terminals 23x. If the difference between the thermal expansion coefficients of the PCB 20x and that of the electronic component 10x is large, for example 10 ppm/K or larger, the electrodes 13x and the terminals 23x are displaced with respect to each other due to the heating, as depicted in FIG. 1B. The amount of the displacement increases with the distance from the center of the electronic component 10x. Accordingly, the larger the electronic component 10x in size in the planer direction, the larger the amount of the displacement of the outermost electrode 13x and the terminal 23x with respect to each other is. After completion of the heating process, the PCB 20x tends to drastically contract with respect to the electronic component 10x. However, the mounting surface side of the PCB 20x resists contracting because the electrodes 13x and the terminals 23x are joined together. On the other hand, the back side of the PCB 20x on which no electronic component 10x is mounted tends to contract to the original size. Consequently, the PCB 20x warps as depicted in FIG. 1C due to the difference in the amount of contraction between the mounting surface side and back side of the PCB 20x. 
As has been described above, one cause of the warpage of a PCB is the difference in thermal expansion rate between electronic components and the PCB. The techniques disclosed in Japanese Laid-Open Patent Publications No. 2006-108460 and No. 2007-88293 attempt to reduce warpage caused by the difference in thermal expansion rate between electronic components and the PCB by increasing stiffness of the PCB or to absorb distortion caused by warpage by using a different material. However, neither of the Japanese Laid-Open Patent Publications discloses control of the differential thermal expansion of electronic components and the PCB, which is the root cause of warpage of the PCB.